FIELD OF THE INVENTION
The present invention relates to an overheating prevention system for an exhaust gas purifier installed in a vehicle or the like.
Conventional devices designed to oxidize exhaust gases of an engine which have passed the exhaust valves and thereby accomplish a high degree of purification of the exhaust gases include a manifold reactor, afterburner, catalytic converter and so on. However, these conventional devices have the disadvantage in that is is difficult in ensure a stable and continuous burning of the exhaust gases under varying various engine operating conditions and that the occurrence of any excessive burning tends to raise the temperature in the burning chamber to over 1000.degree. C. in the case of the manifold reactor and afterburner and over 700.degree. C. in the case of the catalytic converter and such high temperatures tend to cause the destruction of the devices. For this reason, various methods have been attempted with these prior art devices, wherein the exhaust gases are exhausted through a bypass circuit or the supply of secondary air is interrupted in accordance with the detected temperature within the purifier devices thereby restraining the reaction to effect the desired temperature control.
The prior art devices constructed as described above are still disadvantageous in that a delay time of several seconds is involved in the operation of the temperature detectors, bypass circuit, actuator and the like so that if, in the catalystic converter system for example, unburned gases resulting from misfiring of any spark plug are admitted into the purifier, these unburned gases react explosively and it is thus impossible to prevent the occurrence of overheating that involves a rapidly accelerating rise of the temperature in the device, thereby giving rise to thermal destruction or deterioration of the purifier or a reduced purifying performation. These deficiencies have been important obstacles in the realization of a greater safety and practical devices.
It is conceivable that these deficiencies of the prior attempts stem from the fact that the temperature in a purifier is detected to thereby prevent overheating of the purifier. In other words, at the time when the temperature is detected, the unburned gases have already been admitted into the purifier and therefore subsequent actuation of any overheat preventing means can hardly prevent the reaction of those unburned gases which have actually been admitted into the device as well as unburned gases which are about to be admitted into the device. Consequently, with such control methods, no improvement in the response characteristic of the temperature detectors, actuator and the like can prevent unburned gases resulting from failure of the engine to fire from reacting explosively within the purifier and thus giving rise to overheating that causes a rapidly accelerating rise of the temperature in the purifier. It is thus impossible to prevent thermal destruction or deterioration of the purifier or deterioration in the performance of the device.